Porous infusible polymer parts

ABSTRACT

Porous infusible polymer (IP) parts are made by incorporating 0.2 to 10 volume percent organic fibers, preferably with short lengths, into the particulate IP, consolidating the mixture under pressure and optionally heating, and then “burning off” the fibers. After the fibers are burned off the resulting part has porosity in which the pores are elongated, usually retaining the shape of the organic fibers. When these parts are exposed to moisture (which they usually absorb) and then suddenly heated they tend not to blister from vaporization of the water. This makes them useful as parts for aircraft (jet) and other engines and other applications where sudden temperature increase may occur.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. National application Ser. No.12/004,437 filed Dec. 19, 2007, now pending, which claims the benefit ofU.S. Provisional Application No. 60/876,890, filed Dec. 22, 2006; theentire disclosures of the prior applications are herein incorporated byreference.

FIELD OF THE INVENTION

Porous infusible polymer parts which contain small controlled amounts ofporosity, preferably where the pores are elongated, for examplecylindrical, are better able to stand rapid heating without damage afterimbibing moisture.

BACKGROUND OF THE INVENTION

Polymers are ubiquitous in current society, the most common types ofpolymers being used being thermosetting and thermoplastic polymers.However a third type of polymer is also used, the so-called infusiblepolymer (IP). These are polymers that are not crosslinked and so shouldtheoretically be thermoplastic, but their melting and/or softeningpoints are at a higher temperature than their decomposition temperature,so before liquefying as they are being heated, they decompose. Typicallythese types of polymers in commercial use have high decompositiontemperatures, so their maximum use temperatures are usually quite high.Polymers of these types include, but are not limited to, polyimides,poly(p-phenylenes), and polymers composed mostly or all of repeat groupsof the formula

wherein X is NH, N-Phenyl, O (oxygen) or S (sulfur), and Ar isp-phenylene, 4,4′-biphenylene or 1,4-naphthylylene.

Since these IPs cannot be formed as typical thermoplastics, the polymersare often chemically formed, and the resulting polymer, if not already apowder, is ground to a powder. This powder is then subjected to pressureand optionally heat in a mold to consolidate the powder into a shapedpart. Also, optionally, the shaped part can be then sintered to furtherconsolidate the polymer. In many ways this type of shaping process issimilar to that employed in the more familiar powdered metallurgy.

Most polymers, when exposed to moisture, either as liquid water or watervapor (in the air for instance), absorb some amount of water. If thepolymer is then heated rapidly to well above the boiling point of water,the absorbed water will have a considerable vapor pressure and try toescape from the polymer. If the diffusion of the water from the polymeris slow, the internal pressure of the water may cause the formation ofvoids within the polymer (blistering), thereby reducing or destroyingthe usefulness of the polymer part. For instance, if the polymer is apart of a jet engine that stands at ambient temperature in a humidclimate and/or in the rain, the part may absorb a considerable amount ofwater. When the engine is started, sections of the engine, includingwhere such IP parts are located, may be heated rapidly, and as a resultthese parts may blister. Some method of avoiding such blistering whilenot substantially reducing the utility of the part would be desirable.

Porous and foamed polyimides are known; see for instance U.S. Pat. Nos.5,444,097 and 4,780,097, U.S. Published Patent Application No.2006/0039984, and D. W. Kim et al., J. Appl. Polym. Sci. 94:1711-18(2004). In all these references, the pores are more or less spherical(either by measurement or photograph and/or by method of preparation),and in many cases the pores are a substantial volume of the total volumeof the polymer plus pores.

Japanese Patent Application 04-077533A describes a porous materialcharacterized by being made by consolidating a matrix which may be a“resin” which includes “polyimide resin” and “unidirectional” (parallel)carbon fibers which are removed from the composite electrolyticoxidation.

SUMMARY OF THE INVENTION

In one aspect, the present invention is a part comprising an infusiblepolymer, wherein said polymer comprises voids present in a range of fromabout 0.2 to about 10 volume percent, said voids being elongated,wherein a ratio of a longest dimension of said voids to a smallestdimension of said voids is at least 10:1.

In another aspect, the present invention is a process for the productionof a part comprising an infusible polymer having elongated voids, theprocess comprising the steps:

(a) forming a mixture by mixing particles of an infusible polymer with0.2 to 10 volume percent of a second polymer, wherein said volumepercentage is based on the total volume of said infusible polymer andsaid second polymer, and said second polymer is in the form of elongatedpieces wherein a ratio of a longest dimension of said pieces to asmallest dimension of said pieces is at least 10:1;

(b) subjecting said mixture to pressure to form a part; and

(c) heating said part to a temperature to burn off said second polymer;

provided that said infusible polymer has a decomposition point above thetemperature at which the second polymer is burned off.

BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 shows a part made by the presently described process, morespecifically an X-Ray tomograph showing the voids in the part (seeExample 12).

DETAILED DESCRIPTION OF THE INVENTION

Herein certain terms are used, and they are defined below:

The term “infusible polymer” or “IP” as used herein is a polymer that isessentially uncrosslinked but does not melt or soften enough to be meltprocessed—that is, processed in a molten or softened state—below itsdecomposition temperature. Useful types of IPs include polyimides,poly(p-phenylenes), and polymers composed mostly or all of repeat groupsof the formula

wherein X is NH, N-Phenyl, O (oxygen) or S (sulfur), and Ar isp-phenylene, 4,4′-biphenylene or 1,4-naphthylylene. Polyimides arepreferred. Since it is often difficult or impossible to prove by testthat IPs are not crosslinked, they will be considered for the purposesherein uncrosslinked if their indicated chemistry of formation is suchthat one would reasonably believe them, based on such chemistry, to beuncrosslinked.

By “burn off” is meant to remove all or substantially all polymer byheating, either in a chemically inert or chemically reactive atmospherebelow the decomposition temperature of the IP. For example, when heatedto a particular temperature, the second polymer (SP) may depolymerize orotherwise pyrolyze to its constituent monomers or other decompositionproducts. In a chemically reactive atmosphere such as air, the SP may beoxidized by the oxygen in the air to form volatile products such aswater and/or carbon dioxide. In this context, “substantially all” meansthat not all of the second polymer is removed from the fusible polymer,but enough is removed that voids having the proper shape and“dimensions” are formed.

By “elongated” is meant that the ratio of the longest dimension of theitem should be at least 10 times the shortest dimension, preferably theratio should be at least 25, and more preferably at least 100. Thisholds for both voids and pieces of the SP. As referenced herein, theratio is the average for such elongated voids, and does not includevoids caused by incomplete consolidation of the IP. Since this ratio isdetermined by the fiber length and diameter, it is taken as that ratiofor the fibers used in making the composition. If fibers are not used inmaking the composition, the void's average long and short dimensionsshall be determined by X-Ray Tomography (see below).

By “volume percent voids” (porosity) is meant the volume occupied by theSP in the mixture of the IP and SP when forming the porous part,assuming both of these polymers are fully consolidated. This is acalculated number using the following calculation:

${\% \mspace{14mu} {Voids}} = \frac{\left( {{Wt} \cdot {{SP}/{DenSP}}} \right) \times 100}{\left\lbrack {\left( {{Wt} \cdot {{SP}/{DenSP}}} \right) + \left( {{Wt} \cdot {{IP}/{DenIP}}} \right)} \right\rbrack}$

wherein Wt. is “weight of”, and Den is “density of”. If the IP powderalready has other items incorporated into the powder particlesthemselves such as one or more fillers, the density of the IP shall betaken as the density of the particle composition. Similarly if the SPhas other items in the composition, the density of the SP will be takenas the density of that composition.

By a “part” is meant any shaped object. It may be a final shape that isuseful directly, or a “preform”, “blank” or “standard shape” that willbe cut and/or machined into its final shape.

The ratio of the longest dimension to the shortest dimension of the SPpieces or the voids is measured on a number of either of these items,and the results averaged to get the ratio. For example, if the SP piecesare fibers the lengths and diameters of each of the fibers are measured.The length of each fiber is then divided by the fiber's diameter(assuming a circular cross section), and the results of a number ofthese ratios is averaged.

The porous IP part is made by mixing particles of the IP, typically afine powder, with elongated particles of the SP. The mixing shouldpreferably be done so as to obtain a uniform dispersion of the SP in theIP. This mixture is then subjected to pressure in a mold to shape it. Atthis point, pressure may be the only “force” for consolidation to asolid part, but some heat may also be used. At least at the beginning ofthe consolidation, the temperature should not exceed the decompositionpoint of the SP, in order to “imprint” the volume taken up by the SP inthe internal part of the IP part. However, once the IP part shape hasbeen set, if desired the decomposition temperature of the SP can beexceeded. One probably would often not want to exceed the decompositiontemperature of the SP while the part was in mold because excessivepressure could be generated and/or the mold may be fouled by the SPdecomposition product(s). After the part is formed it may be removedfrom the mold and heated (sintered). The sintering can not only removethe SP by pyrolysis and/or chemical reaction (oxidation in air forinstance), but may also assist in densifying the final part. Subject tothe point made in this paragraph, conditions for forming the part fromthe IP particulate can be the same as is normally used and/orrecommended for the IP.

The SP pieces are essentially the “templates” for the size and shape ofthe voids to be formed in the IP. They may be of any elongated shapemeeting the requirements of the SP size and shape. However a preferredform for the SP is a fiber, especially a fiber with a circular crosssection, in other words the latter will form a void in the shape of atube with a (more or less) circular cross section. In this instance, asmentioned above, the ratio of the longest dimension to the shortestdimension for both the SP and the void will be the length of the fiberdivided by its diameter. One reason fibers are preferred is that theymay be readily formed from many thermoplastics, and in many instancesthe fibers are relatively inexpensive.

The SP is a minimum of about 0.2 volume percent, preferably 0.5 volumepercent and more preferably about 1.0 volume percent of the total volumeof the SP and IP. The maximum amount of SP is about 10 volume percent,preferably about 7 volume percent, preferably about 5 volume percent,and very preferably about 3 volume percent of the total volume of the SPand IP present. Any maximum and minimum volume percents may be combinedto form a preferred volume percent range.

In the present porous IPs, the fibers, and hence the pores, arepreferably not parallel, more preferably not substantially parallel, toone another because the fibers are typically mixed with the particulateIP in a random fashion before consolidation. By “substantially parallel”is meant that the long axis of any given random pore is at least a 10°angle to any other randomly chosen pore. Put another way, the averageangle between the longitudinal axes of any two pores is at least 10°.Note however this does not mean that there is no general alignment ofthe fibers (and hence pores), even though not even substantiallyparallel, the fibers and pores may have a preferred orientation.

Preferably the present parts are at least about 1 mm thick in theirsmallest cross sectional dimension, more preferably at least about 2 mmthick.

Second polymers suitable use in the present invention include:polypropylene, polyethylene, acrylic polymer, cellulose acetate, andcellulosic polymers, for example. Other suitable polymers may be knownto one of ordinary skill in the polymer arts, and such polymers wouldnot be outside of the scope of the present invention. There is a classof polymer made to readily depolymerize or pyrolyze cleanly at a giventemperature, for instance some polymers made for masking applications inelectronics. These polymers are also useful herein. These polymers madeto decompose are often (meth)acrylates or copolymers of (meth)acrylateswith other monomers. Of course the particular SPs useful with anyparticular IP will depend on the decomposition temperature of theparticular IP used. The pyrolysis or reaction temperature of the SPshould of course be just below or preferably significantly below the IPdecomposition temperature. Whatever SP is used and whether it is asimple thermal degradation or a reaction (for example oxidation), theless residue from the removal of the SP that remains in the IP part, thebetter.

A preferred type of IP is a polyimide. Polyimides typically are derivedfrom tetracarboxylic acids (or their derivatives such as dianhydrides)and diamines such as pyromellitic dianhydride (PMDA) and diaminodiphenylether (ODA) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) andODA. A typical example of a polyimide prepared by a solution imidizationprocess is a rigid, aromatic polyimide composition having the recurringunit:

wherein R₅ is greater than about 60 to about 85 mole percent p-phenylenediamine (PPD) units and about 15 to less than about 40 mole percentm-phenylene diamine (MPD) units.

The tetracarboxylic acids preferably employed in the practice of theinvention, or those from which derivatives useful in the practice ofthis invention can be prepared, are those having the general formula:

wherein A is a tetravalent organic group and R₆ to R₉, inclusive,comprise hydrogen or a lower alkyl, and preferably methyl, ethyl, orpropyl. The tetravalent organic group A preferably has one of thefollowing structures:

wherein X comprises at least one of —O—, —S—, —SO₂—, —CH₂—, —CH₂CH₂—,and

As the aromatic tetracarboxylic acid component, there can be mentionedaromatic tetracarboxylic acids, acid anhydrides thereof, salts thereofand esters thereof. Examples of the aromatic tetracarboxylic acidsinclude 3,3′,4,4′-biphenyltetracarboxylic acid,2,3,3′,4′-biphenyltetracarboxylic acid, pyromellitic acid,3,3′,4,4′-benzophenonetetracarboxylic acid,2,2-bis(3,4-dicarboxyphenyl)propane, bis(3,4-dicarboxyphenyl)methane,bis(3,4-dicarboxyphenyl)ether, bis(3,4-dicarboxyphenyl)thioether,bis(3,4-dicarboxyphenyl)phosphine,2,2-bis(3′,4′-dicarboxyphenyl)hexafluoropropane, andbis(3,4-dicarboxyphenyl)sulfone.

These aromatic tetracarboxylic acids can be employed singly or incombination. Preferred is an aromatic tetracarboxylic dianhydride, andparticularly preferred are 3,3′,4,4′-biphenyltetracarboxylicdianhydride, pyromellitic dianhydride,3,3′,4,4′-benzophenonetetracarboxylic dianhydride, and mixtures thereof.

As an organic aromatic diamine, use is preferably made of one or morearomatic and/or heterocyclic diamines, which are themselves known to theart. Such aromatic diamines can be represented by the structure:H₂N—R₁₀—NH₂, wherein R₁₀ is an aromatic group containing up to 16 carbonatoms and, optionally, containing up to one heteroatom in the ring, theheteroatom comprising —N—, —O—, or —S—. Also included herein are thoseR₁₀ groups wherein R₁₀ is a diphenylene group or a diphenylmethanegroup. Representative of such diamines are 2,6-diaminopyridine,3,5-diaminopyridine, m-phenylenediamine, p-phenylene diamine,p,p′-methylene dianiline, 2,6-diaminotoluene, and 2,4-diaminotoluene.

Other examples of the aromatic diamine components, which are merelyillustrative, include benzene diamines such as 1,4-diaminobenzene,1,3-diaminobenzene, and 1,2-diaminobenzene; diphenyl(thio)ether diaminessuch as 4,4′-diaminodiphenylether, 3,4′-diaminodiphenylether,3,3′-diaminodiphenylether, and 4,4′-diaminodiphenylthioether;benzophenone diamines such as 3,3′-diaminobenzophenone and4,4′-diaminobenzophenone; diphenylphosphine diamines such as3,3′-diaminodiphenylphosphine and 4,4′-diaminodiphenylphosphine;diphenylalkylene diamines such as 3,3′-diaminodiphenylmethane,4,4′-diaminodiphenylmethane, 3,3′-diaminodiphenylpropane, and4,4′-diaminodiphenylpropane; diphenylsulfide diamines such as3,3′-diaminodiphenylsulfide and 4,4′-diaminodiphenylsulfide;diphenylsulfone diamines such as 3,3′-diaminodiphenylsulfone and4,4′-diaminodiphenylsulfone; and benzidines such as benzidine and3,3′-dimethylbenzidine.

Other useful diamines have at least one non-heteroatom containingaromatic rings or at least two aromatic rings bridged by a functionalgroup. These aromatic diamines can be employed singly or in combination.Preferably employed as the aromatic diamine component are1,4-diaminobenzene, 1,3-diaminobenzene, 4,4′-diaminodiphenylether, andmixtures thereof.

The porous IP may contain materials other than the IP itself. It maycontain materials that IP compositions normally contain such asfiller(s), reinforcing agent(s), pigment(s), and lubricant(s), etc.These may be present when the IP is formed, so that a particulatecontaining the one or more of these materials is produced. Thisparticulate containing the other material(s) in addition to the IP isused in the present process. Alternatively the other materials to beadded to the IP may be mixed in with the IP and SP in the presentprocess and the whole consolidated together. A combination of these twomethods may be used to add different materials to the composition. Ofcourse any other materials meant to be in the final composition shouldbe thermally stable up to the temperature at which the SP is removedfrom the part.

The void containing (porous) parts described are particularly usefulwherein they are heated rapidly (often much) above the boiling point ofwater after having been exposed to water at lower (ambient) temperaturewhich allowed them to imbibe water. Their tendency to blister (formuncontrolled voids) under these conditions is greatly reduced. It isbelieved that the elongated pores of the present parts form pathwayswhich allows the escape of water (vapor) which may form when “wet” partsare heated rapidly.

This makes them useful, for instance, in parts used in (including partsadjacent to) jet engines, internal combustion engines, turbochargers,electrical and electronic parts subject to high temperatures (eitherinternally or externally generated). Even though these parts containporosity, the controlled nature of the porosity and its relatively lowlevel gives parts whose physical properties such as strength andtoughness which usually are not greatly affected by the porosity. Ofcourse jet engines, internal combustion engines, turbochargers, andelectrical and electronic parts subject to high temperatures (eitherinternally or externally generated) may comprise a part comprising theporous IP described herein.

The shape of the voids, and their dimensions, may be measured and“visualized” by using X-ray microtomography, as generally described inA. Susov and D. van Dyck, Desktop X-Ray Microscopy and Microtomography,Journal of Microscopy, vol. 191, p. 151-158 (1998), which is herebyincorporated by reference. FIG. 1, which is a cross section of a partmade as described in Example 12, shows the voids made after thepolypropylene fibers were “burned off”.

All patents and other references described in the examples are herebyincorporated by reference, as if fully set forth herein.

In the Examples, certain abbreviations are used. They are:

-   -   BPDA—3,3′,4,4′-biphenyltetracarboxylic dianhydride    -   MPD—-phenylenediamine    -   PPD—p-phenylenediamine

Example 1

Particles of a polyimide resin comprising 50 wt % of a polyimide basedon BPDA, PPD, and MPD (with a 70/30 weight ratio of PPD/MPD) and 50 wt %of synthetic graphite were prepared according to the method described inU.S. Pat. No. 5,886,129 (e.g., Example 7) and milled through a 20 meshscreen.

Example 2

Polypropylene fibers (˜3-4 denier) were cut to lengths from about 0.5 mmto about 3 mm. These cut fibers, at 1 wt % loading, were dispersed intoresin from Example 1 by combining fiber and resin in a Waring-typeblender and blended at high speed for 15 sec. Test samples in the formof micro-tensile bars were molded according to the method described inU.S. Pat. No. 4,360,626 (esp. column 2, lines 54-60). Specific gravitywas determined. Tensile strength and elongation were determinedaccording to ASTM D 638-03, using an 1122 model Instron®. The crossheadspeed was 0.2 in/sec (5.1 mm/sec) and an extensometer was attached tothe bar during testing to measure elongation. The results are reportedin Table 1.

Examples 3 and 4

Test samples were prepared containing 2 and 4 wt % polypropylene fiberaccording to the method of Example 2. Physical testing results arereported in Table 1.

Comparative Example A

Test samples were prepared from resin described in Example 1 with 2 wt %of polypropylene fiber. Fiber and resin mixing were accomplished by rollmixing overnight, not in a blender. Physical testing results arereported in Table 1.

Comparative Examples B and C

Test samples were prepared from resin described in Example 1, accordingto the method in Example 2 but without the polypropylene fiber, eitherwith or without treatment in the blender. Physical testing results arereported in Table 1.

In Table 1 Specific Gravity is gm/mL, Tensile Strength to break is MPa,and Elongation is percent.

TABLE 1 Fiber Spec Example wt % Blended Grav Tens Str Elongation 2 1 Yes1.6559 91.0 5.5 3 2 Yes 1.6264 76.5 3.1 4 4 Yes 1.5600 71.7 2.3 A 2 No1.6220 56.5 1.1 B 0 No 1.6925 97.9 6.5 C 0 Yes 1.6852 97.9 5.4

Although there is some decrease in physical properties when porosity ispresent, especially when the fiber is not well dispersed, the porositydoes not lead to very large decreases in these properties, especially atthe 1% level.

Example 5

Samples from the preceding examples were conditioned for a thermal shocktest by soaking in 95° C. liquid water for 14 days. The samples werethen thermally shocked by placing them in an oven preheated to 325° C.,350° C., 375° C., or 400° C. for 1 h. After the 1 h heat soak, thesamples were removed from the oven, allowed to cool and then examinedfor the presence of blisters. The presence of blisters as noted under“Observations” in Table 2, below, indicate which samples failed thetest, and the temperature at which the blisters first appear. The testresults are reported in Table 2.

TABLE 2 325° C. 350° C. 375° C. 400° C. Example ObservationsObservations Observations Observations 2 None None None None 3 None NoneNone None 4 None None None None A Small blisters Small blisters SmallSmall blisters blisters B None Small blisters Blistered Blistered CBlistered Blistered Blistered Blistered

Examples 6-11

Other samples were prepared using the method described in Example 2using different fibers at 4 wt % fiber loading. These fibers, which werenominally 3 mm long, were obtained from Engineered Fibers Technology,LLC (Shelton, Conn. 06484, U.S.A.). In order to be considered suitablefor producing controlled porosity in polyimide parts, it must bepossible to mold the parts without blistering during the sintering step.The results for molding of samples with these fibers are reported inTable 3. These results possibly could be changed (improved) by alteringthe heating cycle when the fibers are “burned off”, especially byheating more slowly. These Examples illustrate that a variety of fibers,and of different diameters, may be used to form the pores.

TABLE 3 Example Fiber Material Denier* Result 6 Polyethylene 4 NoBlisters 7 Cellulose Acetate 1.5 No Blisters 8 Polyvinylalcohol 0.3Blistered 9 Lyocell ® Tencel 1.5 No (cellulosic) Blisters 10 Acrylic 0.3No Blisters 11 Acrylic 1.5 Blistered *Denier is the number of grams per9000 meters of a single filament of fiber.

Example 12

By a method similar to that in Example 2, 1.2 weight percent ofpolypropylene fiber was blended with the polyimide. The mixture wasplaced in a mold which was placed in a hydraulic press and compressed at276 MPa. These parts were sintered under nitrogen using a heating cycleof ambient temperature to 400° C. over a period of 59 hours, and thenheld at 400° C. for 3 hours, and then cooled. The parts were thenmachined into final parts. One of these parts was then subjected toX-Ray Tomography, the result of which is shown in FIG. 1, which is froma video of that tomography. The “lines” visible in the Figure are thepores formed by pyrolysis of the polypropylene fiber and are voids inthe polyimide (which was “subtracted out” from the tomograph). A scalemarker is shown in the Figure. This is only a portion of the part, thepolyimide (“solid”) portion of which is not shown, but in FIG. 1 extendsas in the form of a rectangle to the overall periphery of the voidsshown. Note that the fibers appear to have a preferred orientation, butare not substantially parallel.

1. A process for the production of a part comprising an infusiblepolymer having elongated voids, the process comprising the steps: (a)providing particles of an infusible polymer, wherein the infusiblepolymer is selected from the group consisting of polyimides,poly(p-phenylenes), and polymers composed mostly or all of repeat groupsof the formula

wherein X is NH, N-Phenyl, O (oxygen) or S (sulfur), and Ar isp-phenylene, 4,4′-biphenylene or 1,4-naphthylylene; and (b) forming amixture by mixing the particles of the infusible polymer with about 0.2to about 10 volume percent of a second polymer, wherein said volumepercentage is based on the total volume of said infusible polymer andsaid second polymer, and said second polymer is in the form of elongatedpieces wherein a ratio of a longest dimension of said pieces to asmallest dimension of said pieces is at least 10:1; (c) subjecting saidmixture to pressure to form a part; and (d) heating said part to atemperature to burn off said second polymer; provided that saidinfusible polymer has a decomposition point above the temperature atwhich the second polymer is burned off.
 2. The process of claim 1,wherein the infusible polymer is a polyimide.
 3. The process of claim 1,wherein the part is at least about 2 mm thick in its smallest crosssectional dimension.
 4. The process of claim 1, wherein the elongatedpieces are not substantially parallel.
 5. The process of claim 1,wherein the second polymer is one or more of polypropylene,polyethylene, an acrylic polymer, cellulose acetate, or a cellulosicpolymer.
 6. The process of claim 1, wherein the mixture of step (a) is auniform dispersion of the second polymer in the infusible polymer. 7.The process of claim 1, wherein the second polymer is a fiber.
 8. Theprocess of claim 7, wherein the second polymer has a circularcross-section.